Valganciclovir is a mono-L-valyl ester prodrug of the antiviral compound ganciclovir.
Valganciclovir is represented by chemical formula I, wherein Z is, hydrogen, is chemically, a L-valyl ester of 2-(2-amino-1,6-dihydro-6-oxo-purine-9-yl)-methoxy-3-hydroxy-1-propane.

Ganciclovir of Formula II, where P1 and P2 are hydrogen, is disclosed in U.S. Pat. No. 4,355,032.

Ganciclovir inhibits replication of human cytomegalovirus both in vitro and in vivo. When administered orally, Ganciclovir has a very low rate of absorption. Ganciclovir is mostly used as an intravenous infusion.
Antiviral Purine derivatives with an acyclic chain in the 9 position, such as acyclovir (INN name) & Ganciclovir (INN name) have been disclosed in British Patent 1523865 & U.S. Pat. No. 4,355,032 respectively.
Antiviral compounds with better water solubility as compared to ganciclovir have been disclosed in British Patent Application GB 2 122 618, without mention of mono- or bis L-valyl esters of ganciclovir.
Though Bis-L-valyl ester prodrug of ganciclovir with a better bioavailability profile is mentioned in European Patent Application 0 375 329, it does not disclose any details as to utility or preparation for mono L-valyl ester of ganciclovir.
Mono- and diacyl esters of ganciclovir are disclosed by John C. Martin et al. in J. Pharm. Sci. 76(2), p. 180-184. Various mono- and diacyl esters of ganciclovir are disclosed with their methods of preparation. However, L-valyl ester of ganciclovir and their process of preparation are not mentioned in this article.
European Patent Application 0 694 547 A discloses an antiviral compound with improved oral absorption and low toxicity which is a L-monovaline ester derived from 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol and its pharmaceutically acceptable salts.
The process typically described in the said European Patent Application is:

U.S. Pat. Nos. 5,700,936, 5,756,736, 5,840,890, 5,856,481, 5,840,891, 6,083,953, 6,103,901, 6,040,446, 6,215,017, 6,218,568, and US patent application 2002/042424 provide processes for preparation of mono L-valyl ester of ganciclovir of formula I, wherein Z is hydrogen or an amino protecting group or pharmaceutically acceptable salt thereof which involves protection of either of the two hydroxy groups of optionally amino protected ganciclovir and treating the monohydroxy protected ganciclovir of formula II, wherein P1 is an amino protecting group and P2 is hydroxy protecting group with protected L valine of formula III, wherein Z is amino protecting group, to get protected monovalyl ester of ganciclovir of formula IV.

The compound is deprotected to get valganciclovir of formula I or pharmaceutically acceptable salt thereof in crystalline form.
Thus, the above mentioned prior art processes involve various protected and partially protected ganciclovir derivatives as intermediates. For example, U.S. Pat. Nos. 5,700,936, 5,756,736, 5,840,890, 5,856,481 involve divalinate N,O-bistrityl, monocarboxylate-monovalinate, bis(L-valinate) intermediates. While U.S. Pat. Nos. 6,040,446, 6,215,017 & 6218568 involve persilyl guanine or glycerol derivatives as intermediates.
These various above-mentioned intermediates are then subsequently deprotected and converted to valganciclovir of Formula I or pharmaceutically acceptable salt thereof in crystalline form.
Again WO97/27197 discloses a process explained in the scheme:

Patent application WO2005/092891 details a process for the preparation of the optically pure form of valganciclovir or pharmaceutically acceptable salts thereof by using a solvent or a solvent system characterized in that ganciclovir is soluble in it. Examples of such solvents disclosed include dimethylsulphoxide and sulpholane.